Composite material, e.g. where the matrix material is ceramic (such as, for example, silicon nitride, zirconia, alumina, ailicate glasses, magnesia-aluminate spinel) or metal, is advantageously reinforced with ceramic fibers where high strength and stiffness is required.
Polycrystal alumina fibers are not suitable for use in composite materials because they weaken at high temperatures and thus disintegrate during forming, e.g. in hot pressing or hot isostatic pressing.
Alpha-alumina whiskers, which are single crystals, retain their strength at high temperature and thus, unlike polycrystal alumina fibers, are suitable for forming processes and high temperature usage, and therefore are suitable for use in composite materials.
However, alpha-alumina whiskers have not come into commercial usage for reinforcing composites because known processes for their production have disadvantages. In Particular, it is known to form alpha-alumina whiskers from vapor phase reactions and from growth from a melt. Brisbin et al U.S. Pat. No. 3,094,385 and Glass U.S. Pat. No. 3,615,258 teach vapor phase reactions. The Brisbin method requires control over gas flow and control over level of supersaturation (as small changes in supersaturation level can change morphology), and the Glass method requires electrostatic treatment. Labelle et al U.S. Pat. No. 3,650,703 discloses growth from a melt and has the disadvantage therefore of requiring very high temperatures (a temperature slightly above the melting point which is the vicinity of 2000.degree. C. is recited in column 6).
So far there has been no teaching of producing alpha-alumina whiskers from seeded or unseeded alumina gels.
lt has been found herein that alpha-alumina whiskers can be grown from seeded alumina gel in a process which is much simpler than the known vapor phase and melt growth processes.